Thick poles planted in river (5)
I believe the answer is:
dense
'thick' is the definition.
(I know that thick can be written as dense)
'poles planted in river' is the wordplay.
'poles' becomes 'NS' (North and South poles).
'planted in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'ns' placed inside 'dee' is 'DENSE'.
(Other definitions for dense that I've seen before include "Thick, impenetrable" , "Thick and tightly-packed" , "Solid, thick" , "Needs (anag) -- thick" , "Close knit" .)